U.S. Pat. No. 4,397,270 (=JP-A 55-137305) discloses a VVA apparatus. It includes a drive, a control rod with axially spaced eccentric cams, and a pivot structure. The pivot structure supports valve operating (VO) cams for pivotal motion above valve lifters of cylinder valves. Springs are mounted for the VO cams, respectively. Each of the springs biases one of the corresponding rocker cams toward its rest position where the associated cylinder valve closes. Rocker arms operate the VO cams, respectively. The eccentric cams, which are in rotary unison with the control rod, bear the rocker arms, respectively. An axis of each of the eccentric cams serves as the center of drive of the corresponding one of the rocker arms. Cams fixed to the drive shaft operate the rocker arms, respectively. An electronic controller is provided. Sensors on the engine send information on engine speed, engine load, vehicle speed, and coolant temperature to the controller. At a predetermined switchover point, the controller sends a signal to an actuator for the control rod. As the actuator turns the control rod, the eccentricity of each of the eccentric cams with respect to an axis of the control shaft changes. This alters the position of pivot center of the rocker arms relative to the position of pivot center of the VO cams. This causes variation in valve timing and lift of each of the cylinder valves.
According to this known VVA apparatus, operating angle of the VO cam increases when high lift mode is selected. The high lift mode is selected for operation at high engine speeds. The increased operating angle of the VO cam is not suitable for smooth operation of the VO cam and its associated valve lifter during operation at high speed because the VO cam is subjected to increased acceleration.
An object of the present invention is to provide a VVA apparatus wherein operating angle of VO cam is unaltered to improve smooth operation for engine operation at high speeds.